Fix driven solver SurfaceCurrent with no pre-existing excitation

[phdum-a]

Closes #616.

SurfaceCurrent can be used in the driven solver, but don't have any "Excitation" index. Instead they are added to all excitations. There is a bug where if there is no already existing excitation, this fails.

More generally, the Excitation interface was only designed for lumped or wave ports even through there are other times of excitations (like SurfaceCurrent, Dipole, etc) and this should be reworked to be more general and user friendly.

[Sbozzolo]

Do you maybe want to add some of the (very useful) content of #616 (comment) in the documentation?

[phdum-a]

Do you maybe want to add some of the (very useful) content of #616 (comment) in the documentation?

Happy to, but could you let me know what parts in particular? I think that everything I have written is already in the documentation in https://awslabs.github.io/palace/dev/guide/boundaries/ and https://awslabs.github.io/palace/dev/reference/

[Sbozzolo]

Do you maybe want to add some of the (very useful) content of #616 (comment) in the documentation?

Happy to, but could you let me know what parts in particular? I think that everything I have written is already in the documentation in https://awslabs.github.io/palace/dev/guide/boundaries/ and https://awslabs.github.io/palace/dev/reference/

I find that your comment provides insight on how the features should be used, not just a description of the math/interface (e.g, you describe the difference between LumpedPort and SurfaceCurrent from a modeling/physics perspective). I also don't see references to dissipation or even the different normalizations in the boundaries page.